1. Field of the Invention
The present invention relates to a Doppler VHF omnidirectional radio range (DVOR) apparatus that is a radio navigation facility, and relates to a sideband antenna fault detecting method to be used for detecting a faulty sideband antenna in the DVOR apparatus.
2. Description of the Related Art
A device which provides direction information from a reference point, namely magnetic north, to an airplane is in general use. An antenna of the DVOR apparatus is composed of a carrier antenna and 48 sideband antennas which are arranged around the carrier antenna as the center. The carrier antenna radiates radio frequency (RF) signals with prescribed phases having no relation to any direction. The sideband antenna radiates RF signals of which the phases vary in accordance with direction. A receiver mounted on the airplane can therefore ascertain the direction from the reference point, namely the DVOR apparatus, at a receiving point by receiving those two types of signals, and comparing them (refer to, e.g., Jpn. Pat. Appln. KOKAI Publication No. 2-24586).
The RF signals radiated from the sideband antennas are output from a main device of the DVOR apparatus. The output timing of the RF signals output to each sideband antenna is switched by a distributor positioned at a place away from the main device. If a fault such as a failure occurs in any of the sideband antennas, the sideband antenna does not radiate the RF signal then the DVOR apparatus cannot fully function. Therefore, a conventional DVOR apparatus has a sideband antenna fault detection function in order to detect a faulty sideband antenna.
The conventional sideband antenna fault detecting function detects reflected waves from the sideband antennas by directional couplers installed in the distributor in the same number as that of the sideband antennas. By monitoring the reflected waves through an antenna monitor circuit also installed in the distributor, the detecting function can detect a faulty sideband antenna. However, adding this function makes the configuration of the distributor extremely complicated. Thus, problems, such that the overall size of the DVOR apparatus becomes large and its manufacturing cost increases, have been posed. For the detection function, cables supplying power sources to drive the antenna monitor circuit, and cables transferring the monitor results of the reflected waves from the antenna monitor circuit outwardly, etc., are necessary, which exacerbates the foregoing problems. Furthermore, if a fault has occurred in the cable transferring the RF signal between the main device and the distributor, this fact poses another problem that the fault occurrence cannot be detected.
As mentioned above, in the conventional DVOR apparatus, since the distributor has to include directional couplers of which the number is same as that of the sideband antennas and the antenna monitor circuit, this has become the main factor inhibiting a decrease in size and manufacturing cost of the DVOR apparatus. If a fault has occurred in the cable transferring the RF signal between the main device and the distributor, it has been impossible for the conventional DVOR apparatus to detect the fault.